Dietary flavonoids include a broad range of compounds that have been shown to have a variety of health benefits (e.g., antioxidants, antiosteoperosis. anticancers, and anti-aging), which make them attractive for disease prevention and impeding the progress of degenerative diseases in humans. However, these compounds have poor bioavailabilities. The long-term goal of our study is to determine how enteric recycling (Liu and Hu. 2002) contributes to the overall disposition of flavonoids and to the biological activities of flavonoids in humans. The general hypothesis for the present research proposal is that the inhibition of intestinal transporter mediated efflux of conjugated metabolites is more effective than inhibition of intestinal conjugating enzymes in decreasing luminal excretion of flavonoid metabolites and in increasing, local, portal and systemic bioavailability of parent flavonoids. The specific aims are to: (1) determine if intestinal metabolism and subsequent excretion of metabolites is more important than liver metabolism and excretion in a variety of established models using selected flavonoids; (2) determine the main efflux transporters responsible for efflux of hydrophilic flavonoid conjugates using RNA interference or RNAi (e.g., siRNA) and chemical inhibitors, and the rate-limiting step in the cellular excretion of hydrophilic flavonoid conjugates; (3) determine how silencing of a main isoform of UDP-glucuronosyltransferase or sulfotransferases responsible for the metabolism of related representative flavonoids will change the cellular excretion of flavonoid conjugates in the Caco-2 model; and (4) identify chemical inhibitors that are capable of decreasing the efflux transporter activities without affecting the activities of UGT or SULT or vice versa using kinetic methods, and determine if inhibitors of main efflux transporter is more effective than inhibitors of the main conjugating enzyme in enhancing the local, portal and systemic bioavailabilities of selected flavonoids. Through these studies, we will further understand the mechanisms that control the fate of flavonoids following oral ingestion and shed light on the reasons why flavonoids have poor oral bioavailability. These findings may be used to understand the mechanisms of actions of flavonoids and to devise means of increasing their bioavailability in humans. [unreadable] [unreadable]